1. Field of the Invention
The present invention relates to a diaphragm used in an electroacoustic transducer such as a speaker, and also relates to a method of fabricating such diaphragm.
2. Description of the Related Art
In general, a material for a diaphragm has to have a small density, large Young's modulus (high rigidity) and appropriate internal loss. Further, the diaphragm material needs to possess a sufficient resistance to the environment.
A typical-material for a conventional diaphragm is PP (polypropylene) which has an adequate environmental resistance (particularly, water resistance) and large internal loss. Another known material for the diaphragm is a liquid crystal polymer which has high rigidity.
One type of diaphragms has a honeycomb structure to increase rigidity and reduce weight. Another type of diaphragms has a three-layer structure that includes a foam and planar skin layers, which sandwich the foam. Among the conventional diaphragms, the polypropylene diaphragm has a greater specific weight than paper and a relatively small Young's modulus. The liquid crystal polymer diaphragm has a relatively large specific weight and a smaller internal loss than polypropylene. No conventional materials meet all of the above mentioned requirements for the diaphragm.
Under these circumstances, it is ideal that the structure of the diaphragm be modified to meet the physical requirements such as density and Young's modulus, and the material of the diaphragm be selected to meet other requirements. In reality, however, if the diaphragm has to have a three-layer structure, a manufacturing process must include a step of bonding the three layers. This results in an increase of the manufacturing cost.
To deal with the above described problems, the present assignee proposed a speaker diaphragm which has a three-layer structure in Japanese Patent Kokai No. 8-340594. Surface layers of this three-layer structure are non-foamed layers, and the inside layer is the foam. This three-layer structure is fabricated by an injection molding process using resin together with a foaming agent. The present assignee also proposed another speaker diaphragm having a three-layer structure prepared by another injection molding process in Japanese Patent Kokai No. 11-275687. The injection molding process uses resin, which is compounded with a filler of inorganic matters or organic matters, together with a foaming agent. The resulting three-layer structure has an inner foam layer and outer (surface) non-foam layers. These injection molding processes can improve the physical characteristics of the diaphragm without increasing the manufacturing cost. The resulting speaker diaphragm is lightweight, and has large internal loss, high rigidity and high environmental resistance.
However, if the three-layer structure is created from the resin with the foaming agent, the resulting product (diaphragm) suffers from certain deterioration. Specifically, the rigidity of the product is lowered by a heterogeneous material (e.g., polyethylene) used to prevent the foaming agent from dispersing. Further, gases derived from the foaming agent would degrade the appearance of the product. In addition, some limitations are imposed on the resin molding process due to a decomposition (dissolution) temperature of the foaming agent.